All perfect praise be to Allah, the Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
When a person performs ‘Umrah and wants to perform another one, he should end his first state of ihram by shaving or cutting his hair, then depart Makkah, and go to the nearest miqat (place for assuming ihram), like At-Tan‘eem or Al-Ji‘raanah, for example, in order to assume a new state of ihram for the following ‘Umrah. This is because assuming a new ihram for each ‘Umrah is a prerequisite. The Prophet, sallallaahu ‘alayhi wa sallam, commanded ‘Abdur-Rahmaan ibn Abi Bakr, the brother of ‘Aa’ishah to accompany ‘Aa’ishah to At-Tan‘eem in order to assume a new state of ihram (for performing ‘Umrah). If it was permissible to assume the state of ihram in Al-Masjid Al-Haraam without having to leave to the nearest miqat, then the Prophet, sallallaahu ‘alayhi wa sallam, would not have ordered her go to the miqat. Imam Al-Bukhari, Muslim, Ahmad and others narrated the above-mentioned hadith. If a person assumed the state of ihram in Makkah in order to perform ‘Umrah and did not go back to the miqat, then his ‘Umrah is valid, but he does bear the sin for that, and it is incumbent upon him to offer a sacrificial animal for failing to go out to the miqat in order to assume a new state of ihram.
Allah Knows best.